1. Field of the Invention
The present invention relates to a mobile communication system. More particularly, the present invention relates to an apparatus and a method for estimating a channel in a wireless communication system.
2. Description of the Related Art
When determining the capability of a mobile communication system, it is very important that noise power is estimated accurately. Therefore, research and various studies have been performed for estimating a channel with accuracy and for increasing the accuracy by estimating the noise power in various schemes.
FIG. 1 is a diagram illustrating a configuration of a receiver in a mobile communication system according to the related art.
Referring to FIG. 1, the receiver includes a wireless communication unit 101, a conversion unit 103, a first de-spreader unit 105, a second de-spreader unit 107, a third de-spreader unit 109, a channel estimation unit 111 and a compensation unit 113.
First, the wireless communication unit 101 performs a down conversion by mixing an input signal received from an RF antenna with a carrier wave frequency, and then transmits the signal to the conversion unit 103. The conversion unit 103 converts the analog signal received from the wireless communication unit 101 into a digital signal, and adjusts a power of the converted digital signal to fall within an operation range of a modem. The adjusted signal is transmitted to plural fingers corresponding to delays of respective multiple paths.
As shown in FIG. 1, the fingers comprise the first to third de-spreader units 105 to 109, respectively.
The first de-spreader unit 105 restores a pilot symbol, the second de-spreader unit 107 estimates a noise power, and the third de-spreader unit 109 restores a data symbol. Each of the first to third de-spreader units 105 to 109 carries out the despreading on the received signal using an orthogonal code for each channel which is used in transmission.
In general, a Spreading Factor (SF) of data is less than that of the code carrying a pilot signal. If the SF of the pilot signal is 256 chips and the SF of the data is 4, the data is applied with the same channel compensation during 64 symbol intervals.
On the other hand, the estimation of the noise power can be performed by measuring in units of a constant time interval according to an implementation scheme. In the conventional technique, since the second de-spreader unit 107 employs a de-spreading scheme in which the de-spreading is performed using an Orthogonal Variables Spreading Factor (OVSF), after the de-spreading, the original signal component does not remain but thermal noise and interference caused by multiple paths remain. In order to obtain the noise power, each In-phase/Quadrature (I/Q) is squared and added together. The noise power estimation value as described above is used for the channel compensation as well as for reporting the Signal to Noise Ratio (SNR) to a Digital Signal Processor (DSP). Therefore, the accuracy for estimating the noise power is a factor that affects the modem performance.
However, the noise power estimation value estimated using a typical channel estimation apparatus includes the other path interference power caused by the multiple paths as well as the thermal noise components. Therefore, since the affect of the other path interference increases as the signal power is increased, the value is not suitable for the noise power estimation value. For example, in a modem system using an equalizer, if the noise power is estimated using unused OVSF as the conventional scheme, the performance of the receiver may be degraded.
Therefore, a need exists for an improved apparatus and method for estimating a channel accurately by extracting a noise power.